1. Field of the Invention
The present invention relates to a thermal head used in a facsimile or a printer system. More particularly, the invention relates to a thermal head which comprises a substrate which is coated with a heat resistant and electrically insulating resin such as polyimide.
2. Description of the Related Art
A thermal head comprises a ceramic substrate the surface of which is coated with a glazed layer of glass material. On the substrate is formed a plurality of heating elements, electrodes and a protection film to constitute a heating substrate. The heating substrate is secured to a support plate through a double-sided adhesive tape. On the support plate is also disposed a printed wiring board of glass epoxy resin secured to the plate through a double-sided adhesive tape adjacent to the heating substrate. The printed wiring plate comprises an IC circuit formed thereon to drive the heating elements. Each electrode of the heating element and each IC chip element are connected together through a wire by a wire bonding method. Also, a common electrode of the heating element and the circuit pattern of the printed wiring board are connected together through a wire.
Another kind of thermal head comprises a heating substrate on which driving IC elements are directly mounted by a COG (Chip on Glass) method. The heating substrate is mechanically held and supported between the support plate and a cover plate.
On the other hand, a thermal head comprising a substrate made from an insulation material such as ceramic or a metal substrate coated with polyimide resin instead of the ceramic substrate coated with the glazed layer is proposed in Japanese Patent Application Laying Open (KOKAI) Nos. 52-100245, 59-142167 and 60-167574 to conform to the G-IV standard or use in a high speed printer. The thermal head comprising the polyimide layer as a heat insulation layer is advantageous from the standpoint of printing speed and energy consumption.
With respect to the thermal head comprising the electric insulation plate or a metal plate coated with a heat resistant insulation resin such as polyimide, the thermal head is produced in such a way that a plurality of thermal head elements comprising thin film circuit patterns and thick film circuit patterns are formed on a large sized substrate (for example about 45 mm .times.270 mm), after that the substrate is divided to the plurality of thermal head elements. Each thermal head element constitutes the heating substrate of the thermal head. When such a heating substrate is bonded to the support plate of the thermal head, the boundary between the insulation plate or the metal plate of the substrate and the heat resistant insulation resin layer is exposed to the ambient air. Also, the sectional portion of the electrode is exposed to the air as well.
Also, with respect to the thermal head which is individually fabricated from an individual substrate instead of being made from a common large substrate by cutting and dividing the substrate, the boundary between the substrate and the heat resistant insulation resin layer and the electrode section are exposed to the ambient air in the case when the resin layer is disposed only on one side of the substrate.
Such a heating substrate is bonded to the support plate to constitute a thermal head. When such a thermal head is used in a state of high temperature and high humidity, or when the thermal head is tested by a reliability test such as a pressure-cracker test, moisture enters into the boundary between the substrate and the resin layer and/or the electrode boundary, which adversely acts on the thermal head. Sometimes, this also results in that the resin layer is removed from the substrate because the bonding strength of the resin layer is weak.